如何使用XF86VidModeSetGamma（）更改第二台显示器的gamma？

Question

I've written a small function to change the gamma value of my monitor, but unfortunately it changes the gamma exactly of the wrong monitor, where I don't need it.

I've tried nearly everything, but I can't find any working solution - it's always the wrong monitor.

How do I tell it correctly of which monitor it should change the gamma?

Another strange thing is, ScreenCount() always returns 1.

This function should be a workaround for the broken SDL_SetGamma(), which doesn't work on Linux, or at least not with ATi cards. I am loading libXxf86vm.so at runtime, so it's an optional feature and doesn't need to be linked into the app.

It would be also great if someone could explain me, how I can change the gamma of my second monitor with the xgamma command, because exactly the same is happening with that command.

#include <X11/Xlib.h>

typedef struct {
    float red;          /* Red Gamma value */
    float green;        /* Green Gamma value */
    float blue;         /* Blue Gamma value */
} XF86VidModeGamma;

typedef Bool (*XF86VidModeSetGamma)(Display*, int, XF86VidModeGamma*);

int changegamma(float red, float green, float blue)
{
    void *lib;
    XF86VidModeSetGamma f;
    int screen;
    Display *display;
    XF86VidModeGamma gamma;

    lib = dlopen("libXxf86vm.so", RTLD_LAZY);
    if(!lib) goto error;

    f = (XF86VidModeSetGamma)dlsym(lib, "XF86VidModeSetGamma");
    if(!lib) goto error; 

    gamma.red = red;
    gamma.green = green;
    gamma.blue = blue;

    display = XOpenDisplay(NULL);

    if(!display) goto error; 
    screen = DefaultScreen(display);

    if(!f(display, screen, &gamma)) goto error; 
    XCloseDisplay(display);
    dlclose(lib);

    return 0;
error:;
    if(lib) dlclose(lib);
    return SDL_SetGamma(red, green, blue);
}

Answer 1

the XF86-interface to screens is really quite old and doesn't reflect modern multi-screen setups (using xinerama,...).

in any case you should use xrandr to set per-display settings these days. the easiest way is probably to just check the source-code of the xrandr utility itself, which can be used to set per-screen gamma from the cmdline.